1. Technical Field
The present invention relates to a novel atomic layer deposition (ALD) reactor.
The present invention more particularly relates to a novel reactor architecture that allows the density of particles present in the reaction chamber to be considerably reduced.
The invention is advantageously used in the encapsulation of an organic light-emitting diode (OLED) via deposition of a transparent encapsulating film, such as a film of Al2O3.
2. Description of Related Art
At the present time OLED-containing devices are encapsulated in order to protect their sensitive components from gaseous species in the atmosphere (mainly oxygen and water vapour). This is because, if it is not suitably protected, there is a risk that the device will subsequently degrade, chiefly resulting in the appearance of non-emissive dark spots. These dark spots in fact result from the penetration of water vapour into the diode, thereby degrading the interface between the cathode or anode and the organic film(s).
For reasons of cost, and for certain applications, two types of chemical vapour deposition technique are, for the most part, currently used to deposit thin films on OLED devices. These thin films must have a barrier effect, thereby protecting the device from attack by external moisture. Generally, these thin barrier films are oxides, nitrides or oxynitrides, or, if required, thin metal films, the latter not being able to be used when the OLED(s) is(are) top-emitting devices, in which case the barrier films have to be transparent, necessarily.
The first type of technique is called PECVD (plasma enhanced chemical vapour deposition) in which gaseous reactants, also called gaseous precursors, are introduced simultaneously and react in the vapour phase (gas-phase nucleation), and with the surface of the substrate, thereby creating certain aggregates. These aggregates are undesirable because they are liable to lead to particulate contamination of the substrate, and also to increase the roughness of the thin film [1], [2]. The second type of technique is ALD (atomic layer deposition), in which the reactants (precursors) are introduced in alternation, these phases of reactant introduction being separated by phases of purging with an inert gas, such as dinitrogen N2. The adsorption reaction between a precursor and the substrate takes place in a second phase. The principle behind an ALD deposition cycle is schematically illustrated in FIG. 1. The reaction is said to be self saturating because the precursor does not react with itself in the gas phase. Since the precursors are injected in alternation, gas-phase reactions between the precursors are inhibited, thereby limiting the formation of aggregates and preventing the formation of defects in the thin films, these defects usually being called pinholes. If it is assumed that this inhibition effect is enhanced when deposition is carried out at low temperatures, the ALD technique turns out to be by far the most suitable technology for encapsulation of organic devices because, intrinsically, it creates little particulate contamination in comparison with the PECVD technique, and, furthermore, it allows completely conformal depositions to be produced.
Nevertheless, the inventor has observed that particles are still generated and that therefore there is a residual particle density in reactors used to carry out ALD processing. The graph illustrated in FIG. 2 shows the results of a particulate contamination test as a function of time, as measured by the inventor in a cleanroom for an ALD reactor used to deposit an Al2O3 encapsulation film on an OLED substrate. More exactly, the reactor was that sold by Cambridge NanoTech under the trade name Savannah. The measurement apparatus used was the surface defect detector sold by Kla-Tencor Corporation, Tencor Instruments under the trademark Surfscan®. The graph shows the inevitable presence of particles. At the present time, in practice, measures are taken in order to limit the density level, i.e. to below a preset threshold S in the number of particles per cm2 (Nd): typically S is chosen to be lower than 300/cm2 as illustrated in FIG. 2. Such a threshold must be set because, as indicated above, particles are disadvantageous when it is desired to deposit defect-free oxide barrier films. The peaks in particle density seen in FIG. 2 thus correspond to periods in the operation of the ALD reactor immediately before its maintenance: in fact, chaotic creation of particles in the reactor after many processing cycles is inevitable because deposits produced on the walls bounding the reaction chamber delaminate when the thicknesses of said deposits become too great. Thus complete and regular maintenance of ALD reactors proves to be necessary. This maintenance is elaborate and expensive, because it involves a complete clean of the machine in which the reactor is incorporated, especially via sandblasting and chemical etching operations.
The general aim of the invention is to overcome at least some of the drawbacks of the prior art and therefore to provide a solution that allows the density of residual particles liable to reach a substrate on which it is desired to produce an ALD film to be reduced or even eliminated.